1. Field of the Invention
The present invention relates to a process for the preparation of a basic alkaline earth metal salt of a blend of organic carboxylic acids, a salt thus prepared and to oil compositions containing such a salt.
2. State of the Art
The use of alkaline earth metal salts of organic carboxylic acids as additives for lubricating oil compositions is known. The said salts have a dispersant property so that they, when applied in such composition, ensure that the inside of engine cylinders remains clean and that deposition of carbonaceous products on pistons and in piston grooves is counteracted, so that piston-ring sticking is prevented.
It is also known to prepare basic (or overbased) alkaline earth metal salts of such acids. The overbasing provides an alkaline reserve which, when applied in lubricating oil compositions, reacts with and neutralises acidic compounds formed during the operation of the engine in which the composition is applied. Hence, sludge which may arise, is maintained dispersed due to the dispersant property of the salt while acids which would enhance sludge formation are neutralised.
In British patent specification No. 786,167, a process for the preparation of basic salts is described in which an organic acid is reacted with an excess of an alkaline earth metal oxide or hydroxide in an oil and subsequently carbon dioxide is passed through the reaction mixture to yield basic salts. As suitable acids are mentioned substituted or unsubstituted aliphatic, cycloaliphatic and aromatic acids, comprising carboxylic acids, sulphur-containing acids, phosphoric acids, thio-acids, phenols and partial esters of sulphur- and phosphorus-containing acids.
In the technical field there is a desire to use products with a basicity as high as possible, i.e. the relative proportion of the organic acid rest in the basic salt is as low as possible. The reason for this is that the costs of the product are mainly incurred by the costs of the organic acid.
The basicity of the products prepared according to the prior art process can amount up to a value of the basicity index of 10, the basicity index (BI) being defined as the equivalents ratio of the total of alkaline earth metal to the total of organic acids. It was found that in the preparation of products having such a high BI or an even higher BI using carboxylic acids a pronounced tendency to gelation occurs thereby severely hindering the handleability of the products. Another problem was that when using certain acids only relatively low BI values were obtainable. It has now been found that when a blend of two specific types of carboxylic acids is used, a high BI value can be obtained without incurring gelation problems.